Re: what evolutionary change occured in the brow ridge of humans?

Well , that is a good and tough question.  Evolution is a very tricky thing 
to speculate about.  For instance, why did we develop 5 fingers instead 
of 4 or 6, or what was the appendix originally for?  The answers to such 
things aren't very clear-cut, at least not yet.  But that doesn't mean we 
can't at least develop a few ideas about what might have happened.  

So let's take a look at the brow ridge and first think of some ideas of 
what it's biological or physical purpose is.  In the case of a protruding 
brow ridge, it might be used for protection of the eyes.  If the eyes are 
set far back in the skull, there is a less likely chance that something can 
reach the soft tissue of the eye and cause it damage.  It might have even 
helped shield the eye from the glare of the sun.  It may also be that large 
eyes are better for seeing prey or anything for that matter, and that 
larger eye size may have started to evolve before a larger brain.  That 
would at least account for a protruding brow ridge in early humans.  
Another characteristic of early humans is that the brain case sloped back 
more away from the eyes.  It you look at pictures of chimpanzees and 
gorillas, you can still see this.    

With that in mind, we now have to wonder, why don't modern humans also have 
protruding brow ridges to the extent of our ancestors?  Well, it seems that 
having eyes set further back could still be a good thing, so that would 
build a case for still having larger brow ridges.  Also, the sun is still 
something we shield our eyes from.  But we do have larger brains, which may 
have caught up with our large eyes.  Also, increasing cranial capacity 
changed the skull to a more efficient shape for a larger brain.  So that 
may be an answer.  Or it could be something due to mate selection.  Maybe 
early humans preferred others with less pronounced brows.  Unfortunately 
there is no real way to test for these theories because the results of 
evolution take so long to turn out.  I would guess it may have been a 
combination of the latter three ideas.  If you are more interested I would 
do a search on "anthropology of early humans", or "evolution of cranial 
capacity" and see what you get.  Of course, a better alternative is to pay 
a visit to a local anthropology museum.  The exhibits and people there 
would help you a lot.  Good luck.